An essential portion of a popular form of thermal overload relay is shown in FIG. 1. As shown, the main components are all included in the insulated housing 1. These include the bimetallic element 2 on which are wound the heating elements (not shown) in which flow the control current, a shifter 6 which engages the free end of the bimetallic element and is adapted to move in the direction of arrow P along a groove in the housing, a switching assembly 3 which is coupled to one end of the shifter 6 and a contact assembly 4 which is opened or shut by the switching assembly through a reversing assembly 5.
The switching assembly 3 includes a temperature compensating element 8 having a leading end portion to which is attached an adjusting screw 7. The adjusting screw 7 engages one end of the shifter 6. The temperature compensating element 8 is integrated with a release lever 9, which is able to turn on the hinge point X. An adjusting link 10 is interposed between the adjustment knob or dial 12 and the release lever 9 which is enabled to turn about the fulcrum W. A spring 11 has one end held by a tongue 10a of the adjusting link 10 and its other end held by the housing for urging the head 10b of adjusting link 10 against the eccentric pin or cam 12a of the adjusting dial 12.
The reversing assembly 5 includes a moveable plate 16 which supports an insulating member 17 having one end inserted in a V-shaped groove 13a in the terminal plate 13 and the other end attached to a normally-opened side moveable contact 15 of the contract assembly 4. A contact controlling spring 14 is connected under tension between a saw-tooth groove 13b in the terminal plate 13 and the moveable plate 16 for reversing the plate 16 when its operating line crosses the V-shaped groove 13a (located at point Z) supporting the moveable plate 16.
The contact assembly 4 includes the moveable plate 16 and the terminal plate 13, which are also part of the reversing assembly, the insulating member 17 for driving the normally-opened side moveable contact 15, a normally-closed side moveable contact 18, a normally-open side moveable contact 20 which is arranged to face the normally-opened side moveable contact 15 and is fixed on the housing by terminal plate 19, which is adapted to be driven during operation of the moveable plate 16 by the insulating member 17, and a normally closed side fixed contact 22 which is arranged to face the normally closed side moveable contact 18 and is fixed on the housing by terminal plate 23.
In this relay, when an overcurrent flows in the heating element of the temperature sensitive bimetallic element 2, the latter bends and shifter 6 is moved in the direction P by movement of the free end of the element 2. When the shifter 6 moves, adjusting screw 7 which is attached to the leading edge of the temperature compensating bimetallic element 8 is pushed so that the release lever 9 is turned clockwise on the hinge pin X. As a result, the spring 14 of reversing mechanism 5 has its portion 14a moved in the direction of arrow Q. At this time, the initial turning angle of the release lever 9, with which element 8 is integrated, is set at a predetermined position by the movment of the hinge point X by the adjusting line 10, according to the setting of the adjusting dial 12. When the turn of the release lever 9 proceeds so that the operating line of the spring 14 passes its neutral point, the moveable plate 16 is reversed on the point Z from the shown position and the relay closes.
In this relay, the moveable plate 16 may have unintended variations in its reversing point or may fail to operate as a result of positioning variations of the adjusting dial 12, the bimetallic element 2 or size variations in the component parts. To compensate for those variations in fixing the operating point, a change in the adjusting screw 7 is used. For example, if a gap is formed in the desired contact between the shifter 6 and the temperature compensating element 8 because of these variations, even when overcurrent is flowing through the heating element, the operating line of the spring 14 may fail to cross its neutral point and the moveable plate not be reversed. In this case it is the practice to insert a screw driver in an opening 24 in the housing to adjust the adjusting screw 7 to eliminate the gap and to set operation at the desired point. Alternatively, if the turning operation of the moveable plate is too advanced because of unintended variations, the desired operating pont can be re-adjusted by loosening the adjusting screw. Since the adjusting screw is attached to the leading end of the temperature compensating element 8, the relay tends to be sensitive to vibrations. Moreover, since the adjustment of screw has to be conducted in the direction of movement of shifter 6, the opening 24 needs to be formed in the side of the housing and the strength of the sidewall is reduced. Moreover, to provide a side opening in a manner to prevent the entrance of foreign matter increases production costs. The present invention seeks to improve on these shortcomings of a relay of the kind described.